This is a list I have been updating on the Shadowdark discord server since 6/16/2023

Particularly Important
The Shadowdark RPG 3rd-party publisher license FAQ
Shadowdark RPG Creator Kit

Generally Awesome
Bre’s Zine Resources
Explorers Design blog.
Pulp University – A great Appendix N resource
“We read the Bloggies is a podcast that reads OSR blogs nominated for the bloggie awards. The Gameable tag on their episodes is a treasure trove of useful resources.” – Samuel

Templates
Explorers Design Typesetter Template
Explorers Design Classic Starter Template
Sersa Victory Shadowdark template
Standardcat Template Pack by Luka Rejec
Affinity Publisher Zine Templates by Metaparadox
LaTeX zine template
Scribus Zine Templates by Metaparadox
A template for a no-cut alternative to the pocket mod, this one allows for a four-panel spread when open and a hidden 8-panel spread.
InDesign source files for Cairn Second Edition

Assets
A free asset pack to indicate no AI-generated content.
Yochai Gal’s public domain art repository
Kim Holm’s free art
Freepik: Download Free Videos, Vectors, Photos, and PSD
Elven Tower’s CC-BY maps
A search engine for digitized museum/gallery peices, mostly public domain
Seedling’s modified public domain images
greygnomepress Free Art Assets
Unsplash hosts images, both illustrations and photos, that can be used freely even for commercial purposes.
Public Domain Fantasy Artwork Sources
K. M. Alexander’s Fantasy Map Brushes
Public domain fonts
Fontesk – some fonts free for commercial use
Elstob-font: A variable font for medievalists
Fonts that evoke specific eras
Legacy of Defeat has made some cool fonts free

Tutorials
Public Domain art preparation: monochrome – Vladar’s Blog
Public Domain art preparation: in color
How To Do ZineQuest: A Step-by-Step Guide
How to layout a medieval village
An extensive and well-organized guide to typography
Beginning Lino-cut: Simple art for RPGs
Sourcing Unique (and Free) Art in 2024 | The Grinning Rat Newsletter
A Brief Study of TSR Book Design by Sine Nomine
Information Architecture in The Castle Automatic
An in-depth lesson on dungeon design from the creator of His Majesty the Worm.
Sersa Victory’s cyclic dungeon generation
40+ hours of art lessons from a goblin
A brief post on how to edit your TTRPG material
Tutorial on choosing colour palettes
Gus L has some helpful blog posts on OSR design theory
As well as this blog post on the process of publishing adventures
Yochai Gal posted a set of module writing tips for people starting out.

Software & Tools
Affinity 3 is free – tools for creating & editing images as well as layout/publishing
Scribus – free open-source publishing software
Markdown reprocessor for rendering rpg tables from unformatted lists
McDie by Gengelstein – A graphical way to explore dice, card, and tile probabilities
Edit e-book files, including troubleshooting tools
Zine printing layout tool for PDFs
Photopea is a free browser-based image editor
Pixlr.com is a freemium suite of image editing tools that notably has a tonne of social media ready templates.
A tool for generating a colour palette from an image
Colour palette tool for cartography

Promoting
How to start a blog, and sixteen pointers for a successful blog
List of RPG reviewers open to submissions
OSR blog centralization
An intro to Web Revival,